1. Field
Aspects of this invention relate generally to optical fiber communication, and, more specifically, to an optical amplifier and to a method for amplifying an optical input signal.
2. Description of Related Art
Optical amplifiers, such as rare earth doped fiber amplifiers, are frequently found in fiber-optic communication systems and networks. The cable television industry, for example, provides communication of information (for example, audio, video, multimedia, and data) between a headend and a plurality of consumer devices at least in part via a fiber-optic network—the headend typically transmits information in an optical format, using one or more fiber optic links, and consumer devices may also generate information that may be converted into an optical format for transmission to the headend.
Passive optical networks (“PONs”) are increasingly being used to deliver cable communication services to consumers at affordable prices. A PON architecture is one in which active components are located either centrally (for example, at the headend), or locally (for example, at consumer locations), while passive components are disposed in between. Single optical amplifiers capable of delivering power in a range of about 600 mill Watts (“mW”) to 3 Watts, which can each serve several-hundred customer locations, are desirable to overcome losses of passive components in a PON.
Optical amplifiers that generate suitable powers, however, are often difficult to reliably achieve using conventional rare earth doped fiber technologies. A conventional erbium-doped fiber amplifier architecture 10 is shown in FIG. 1. Amplifier architecture 10 features two stages of erbium-doped fibers 12, 14. To provide gain to erbium-doped fibers 12, 14, fibers 12, 14 are pumped optically by pumps 16. Pumps 16 are coupled to wave division multiplexers 18. Erbium-doped fibers 12, 14 and wave division multiplexers 18, however, are often unable to handle higher pump powers, restricting an efficient output power 20 of amplifier architecture 10 to about 500 mW. In addition, as output power 20 increases, the optical components of amplifier architecture 10 should be qualified for high power, increasing the cost and reliability of the amplifier. Further, failure of any particular pump in amplifier architecture 10 may lower output power 20, which in turn may lower the power in an entire downstream network, affecting multiple consumers. Still further, having a number of pumps and fibers in a serial configuration may cause the amplified wavelength range to shift to longer wavelengths, for example, 1560 nanometers, which may be undesirable in many applications.
The use of cladding pumped technology, in which an ytterbium fiber laser (915 or 975 nanometer pump) is used for pumping in an erbium-ytterbium amplifier, may be suitable for some applications. Cladding pumped technology alone, however, is not currently as developed or reliable as conventional erbium-doped fiber technology, and also requires the use of special pumps and components, increasing amplifier cost. Moreover, a single component having the serial architecture illustrated in FIG. 1 may still not reliably generate up to 3 Watts of output power.
There is therefore a need for a reliable, low-cost, easily configurable, single-component optical amplifier capable of producing at least about 600 mW—and in some variations up to 3 Watts—of output power.